1. Field of the Invention
This invention relates to electronic circuits used to regulate a voltage, and, more specifically, to electronic circuits used to regulate Vcc voltages in an automobile.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in the automobile industry. It is common in the automobile industry for the automobile to have a battery which is used to provide electrical power to the automobile when the engine is not running. The battery also provides the power necessary to start the motor of the automobile. Once the motor is started, either an alternator or a generator provides the electrical voltage necessary to recharge the battery. However, when a vehicle's battery cable is disconnected from its battery when the engine is running, the voltage on the battery cable can become excessive and potentially damage any device connected to it. This condition is referred to as "load dump". During a load dump condition, the voltage on the batteries cable may reach 60 or more volts. Therefore, it is desirable to have all circuits which are connected to the battery circuit to be able to withstand a high voltage load dump condition.
Additionally, market pressures and government constraints are motivating automobile manufacturers to increase the fuel efficiency while decreasing the emissions of automobiles. The market forces are also requiring that automobiles continue to improve their reliability and decrease their costs. The increasing use of electronics for ignition control systems and the like to accomplish these goals is well known in the industry. It will be appreciated by persons skilled in the art that the increase of electronics requires an increase in the use of voltage regulation and pre-regulation circuits to provide a steady and constant voltage to the electronics on an automobile.
Referring now to FIG. 1, a voltage regulator as known in the prior art will now be described. In this circuit, a regulated Vcc voltage is produced from an unregulated battery voltage Vbatt. In general, the circuit can be thought of as having a current bias circuit, a pass element, and a regulation circuit.
The current bias circuit is made from resistors 2, 8, and 16, diodes 4 and 6, transistors 10, 12 and 14. Their operation can be summarized as generating a bias current at the base of transistor 38 which is used by the voltage regulation circuit.
NPN bipolar transistor 46 is the pass element of the voltage regulator. It is understood in the art that the pass element controls the output current of the voltage regulator as a function of the regulation circuit such that a constant output voltage is maintained.
The regulation circuit consists of a band gap circuit and a voltage step up circuit. The bandgap circuit includes resistors 18, 24, 26, and 28, and transistors 20, 22, 30, and 32. In the band gap configuration, a thermally stable voltage is generated at the base of transistors 22 and 32, as is known in the art. The regulation circuit also includes the voltage divider circuit created with resistors 48 and 50. The scaled voltage from the voltage divider is fed back to the bandgap circuit to increase or decrease the current output of pass transistor 46 in response to the output voltage decreasing or increasing, respectively, as is known in the art.
In an automobile application, the Vbatt voltage is typically 12 volts and the Vcc voltage is typically around 5 volts. However, Vbatt can rise to over 60 volts under the load dump conditions described above. The typical prior art solution to handle the load dump condition was to use a pass transistor which can handle high voltage conditions. However, this prior art solution restricts the integration process technology to a high voltage process.
Therefore, it is an object of the invention to have a voltage regulator which can handle a load dump condition but which can be made using a low voltage process. These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read with the drawings and appended claims.